The present invention relates generally to integrated circuit (IC) designs, and more particularly to a temperature-sensitive current source for reliably generating current based on its temperature.
A DRAM device must be constantly refreshed in order to retain data. It is known by those skilled in the art that the data refresh rate depends upon the temperature of the DRAM device. FIG. 1 illustrates a graph 100 showing a relationship between the temperature of a DRAM device and its required data refresh time, which is reversely proportional to the data refresh rate. The x-axis of the graph 100 represents the temperature of a DRAM device, and the y-axis represents the time required to refresh the data stored in the DRAM device. Area A represents the time-and-temperature coordinates that should not be used for data refresh, due to reliability concerns. Area C represents the time-and-temperature coordinates that should not be used for data refresh, due to power consumption concerns. Area B represents the time-and-temperature coordinates that are acceptable for data refresh designs. As shown in the figure, as the temperature of the DRAM device increases, the acceptable data refresh time decreases, meaning that the data refresh rate needs to be increased.
An increase in data refresh rate leads to a higher power consumption. In order to optimize the tradeoff between power consumption and data retention, it is desirable to design the DRAM device in a way that it refreshes data at a lower rate as its temperature is low, and at a higher rate as its temperature is high. A temperature-controlled oscillator is typically implemented in the DRAM device to adjust the data refresh rate based on the temperature. The temperature-controlled oscillator typically includes a conventional temperature-sensitive current source that supplies current with its amount depending on the temperature thereof.
The conventional current source design typically utilizes one or more transistors operating in a sub-threshold region in order to provide a high current variation in response to a change of temperature. However, because the conventional current source operates in a sub-threshold region, it may be particularly susceptible to process variations, and therefore suffers from reliability issues. This renders the conventional current source impractical or unsuitable for use by DRAM devices.
As such, it is desirable to design a temperature-sensitive current source that can generate current responses to a change of temperature in a reliable manner.